This invention relates to an improved apparatus for drawing glass optical waveguide filaments minimal diameter variation.
Optical waveguide filaments are a promising transmission medium recently developed for use in optical communication systems. Because of optical characteristics including but not necessarily limited to purity of materials and lack of light scattering mechanisms, e.g. core-cladding interface imperfections, low-loss light propagation over long distances is now possible. Optical waveguides suitable for use in optical communication systems can be formed by method such as those disclosed in U.S. Pat. Nos. Re. 28,029 issued to D. B. Keck et al. and 3,823,995 issued to L. L. Carpenter.
It has been found that during the process of drawing certain optical waveguide filaments, diameter variations increased in magnitude after a portion of the draw blank had been drawn even when diameter monitoring and control apparatus was employed. While drawing filament from a blank capable of providing more than about 4 km of filament, the variation in filament diameter typically increased from about .+-.1 .mu.m during the drawing of the first section of filament from the blank to about .+-.3 .mu.m by the time the last km of filament was being drawn. More specifically, little or no increase in diameter change would occur until about 10 cm of blank remained. Thereafter, as the blank decreased in size below about 10 cm, the diameter variation would continue to increase to a maximum of about .+-.3 .mu.m.
For various reasons it is desirable to maintain the diameter of optical waveguide filaments as close as possible to a predetermined value. As discussed in the publication "Mode-Dependent Attenuation of Optical Fibers: Excess Loss" by R. Olshansky et al., Applied Optics, Vol. 15, p. 1045-1047, April, 1976, diameter variations have been found to produce losses in optical waveguides. Moreover, when the ends of two optical waveguides are coupled together, the diameters thereof should be substantially the same to maximize the coupling of light therebetween.